Phosphoric acid

Phosphoric acid is a colourless, odourless, inorganic compound with the chemical formula H₃PO₄. It is commonly found as an aqueous solution containing about 85%. Phosphoric acid has many applications, including producing fertilisers, pharmaceutical products, and skin care products.

1.0Introduction

Phosphoric acid is colourless, odourless and a weak inorganic acid. It is typically found as a liquid with a concentration of up to 85%, but it can also exist as a white solid crystal in its pure form. The IUPAC name for phosphoric acid is orthophosphoric acid.

The conjugate base of phosphoric acid is dihydrogen phosphate. In its crystal form, phosphoric acid has a monoclinic structure, and its molecular shape is tetrahedral. It is non-toxic and non-poisonous at low concentrations but can be hazardous at high concentrations

2.0Structure of phosphoric acid 

Phosphoric acid is represented by the formula H₃PO₄ or PO(OH)₃. The structure of phosphoric acid (H₃PO₄) consists of a central phosphorus atom (P) bonded to one oxygen atom (O) with a double bond and three hydroxyl groups (-OH) with single bonds.

Additionally, one oxygen atom is double-bonded to the central phosphorus atom.

This arrangement allows the phosphoric acid molecule to donate up to three hydrogen ions (H⁺), making it capable of acting as a triprotic acid.

3.0Physical Properties of Phosphoric Acid

Phosphoric acid is a colourless, odourless, and viscous liquid. In its solid form, it crystallises quickly due to its strong affinity for water molecules, making it impossible to separate from water completely. As a result, in its concentrated form, phosphoric acid appears as a syrupy, colourless liquid.

• Acidic Nature: It turns blue litmus paper red, indicating its acidic nature.
• Response to Heat: When exposed to a red-hot heat source, phosphoric acid transforms into a glass-like, transparent, yet brittle substance.
• Molecular Weight: 97.994 g/mol.
• Density: 1.88 g/cm³.
• Boiling Point: 158 °C.
• Melting Point: 42.35 °C.

4.0Chemical Properties of Phosphoric Acid

• Triprotic Acid: Phosphoric acid (H₃PO₄) is a triprotic acid, meaning it can donate three hydrogen ions (H⁺) in solution. The dissociation process is represented as:

H_3​PO₄​ ​+H_2​O ⇋ 3H⁺+PO₄³⁻​

• Heat Action: When heated, phosphoric acid can form dimers, trimers, and longer polymeric chains, such as polyphosphoric and metaphosphoric acids.
• Reducing Agent: Phosphoric acid can act as a reducing agent, participating in dehydration reactions to form various phosphorus compounds.
• Reaction with Bases: Phosphoric acid undergoes neutralisation reactions with bases, forming three types of salts. For example, when it reacts with sodium hydroxide (NaOH), it forms different salts such as Na₂HPO₄, NaH₂PO₄, and Na₃PO₄. One such reaction is:

NaOH + H_3​PO₄​ → NaH₂PO₄ + H_2​O

• Acidity: Phosphoric acid is a weak inorganic acid that can irritate the skin and eyes with prolonged exposure. It can dissociate to give different phosphate ions:
• • Removal of one H⁺ ion forms dihydrogen phosphate (H_2​PO₄⁻).
  • Removal of two H⁺ ions forms hydrogen phosphate (HPO₄^2-).
  • Removal of all three H⁺ ions forms phosphate ion (PO₂^3-).
• pH Value: The pH of phosphoric acid ranges from 1.08 to 7.00, depending on its concentration. Higher concentrations result in a lower pH and greater acidic strength. At low concentrations, phosphoric acid can also exhibit basic behaviour. 

5.0Preparation of Phosphoric Acid

Phosphoric acid is a versatile chemical with many applications and is typically prepared using two primary methods: the Wet Process and the Thermal Process.

1. Wet Process

• Overview: This method produces phosphoric acid from the naturally occurring mineral fluorapatite, which contains phosphate.
• Reaction: Fluorapatite reacts with concentrated sulfuric acid (H₂SO₄) and water (H₂O) to form phosphoric acid, calcium sulfate (gypsum), and hydrogen chloride (HCl).
• Chemical Equation: Ca₅​(PO₄​)3​Cl + 5H_2​SO₄​+ 10H_2​O → 3H₃​PO₄ ​+ 5CaSO_4​⋅2H₂​O + HCl
• Purification:
• • The side products and impurities are removed through filtration and evaporation.
  • The resulting phosphoric acid is then concentrated using vacuum distillation to yield superphosphoric acid with a 56-70% P₂O concentration₅.
  • Usage: The impure product from the wet process is typically used in fertiliser production without further purification.

2. Thermal Process

Overview: This method produces a purer form of phosphoric acid by burning phosphorus rock with coke at high temperatures (around 2000°C) in the presence of air.

Steps:

• • Formation of Phosphorus Pentoxide:
  • • Phosphorus (P₄) reacts with oxygen (O₂) to form phosphorus pentoxide (P₂O₅.).
    • Chemical Equation: P₄ + 5O₂ → 2P₂O₅​
  • Hydration of Phosphorus Pentoxide:
  • • Phosphorus pentoxide is then hydrated with water to produce phosphoric acid.
    • Chemical Equation: P₂O₅ + 3H_2​O → 2H₃PO₄
• Purity: The phosphoric acid obtained via the thermal process is highly pure, typically 75-80% purity.
• Additional: Condensed forms of polyphosphoric acids can also be produced by passing steam into the burner, with the gaseous phosphorus oxide being absorbed in a hydration tower to form phosphoric acid.

6.0Uses of Phosphoric Acid

Phosphoric acid is versatile and used across various industries for different applications:

• Rust Removal: Phosphoric acid removes rust by reacting with iron oxide to form a protective layer, thereby preventing further rusting.
• Soft Drinks: In the food and beverage industry, phosphoric acid regulates acidity and enhances flavour, providing a distinctive tart taste to soft drinks and other products.
• Fuel Cells: Phosphoric acid fuel cells (PAFCs) use liquid phosphoric acid as electrolytes. This enhances the cell’s stability and performance, offering an efficiency range of 35%–45% and cost-effectiveness.
• Pharmaceuticals: It is utilised in the pharmaceutical industry to help regulate pH levels in drug formulations.
• Fertiliser Production: Phosphoric acid is crucial for manufacturing fertilisers and supplying essential phosphorus nutrients to plants.
• Dentistry: Dentists use phosphoric acid as an etching solution for cleaning teeth before applying sealants or other treatments.
• Vitamins and Supplements: It plays a role in the production of vitamins and dietary supplements.
• Daily-Use Products: Phosphoric acid manufactures mouthwash, detergents, soaps, and various skincare products.